The invention relates to a screw tightening power tool which comprises a rotation motor, a power supply means connected to the motor, an output shaft, a torque responsive release clutch including a driving half and a driven half and coupling the motor to the output shaft, an activating rod coupled to the power supply means, a latch means associated with one of the clutch halves and cooperating with the activating rod, and a cam means associated with the other of the clutch halves and operatively connected to the latch means for making the latter shift from an activating rod supporting position to an activating rod releasing position as the transferred torque has reached the actual preset release torque level of the clutch.
A primary object of the invention is to accomplish an improved shut-off initiating mechanism in a power tool of the above related type.
The problem to be solved by the invention is to avoid the undesirable influence of the cam mechanism on the torque load acting on the release clutch. In a previous power tool of the above type, the friction forces developed by the cam means acting on the latch means adds an undesireable, non-constant load on the release clutch. That resulted in a scattered output torque from the tool.
In U.S. Pat. No. 3,766,990, there is described an automatic screwdriver in which the shut-off initiating cam means is of the rotating type formed in one piece with the driven clutch half. By that arrangement, the friction forces developed by the cam means and loading the release clutch are reduced to some extent. Still, the cam means of this known tool is arranged to perform its shut-off initiating action on the latch simultaneously with the cam lobes of the clutch reaching their maximum torque transferring positions. This means that the maximum torque transferred by the clutch is determined both by the clutch itself and by the friction forces developed by the shut-off initiating cam means. In U.S. Pat. No. 4,154,308 there is shown a screw driving tool of the same type as in U.S. Pat. No. 3,766,990 with the difference that the shut-off initiating cam means comprises a cam apex that is sidewise displaced. The purpose thereof is to delay the shut-off initiating movement of the latch until the clutch balls have passed the apex of the cam lobes so as to avoid a situation where the latch is locked on top of one of the apexes and, thereby, blocks reopening of the air supply valve. By this delayed cam-latch engagement, however, the friction forces developed between the latch and the shut-off initiating cam means are prevented from adding to the maximum torque set on the release clutch. This auxiliary effect, which is not recognized in the specification of U.S. Pat. No. 4,154,308, is obtained in one direction of rotation only.
The above problem is overcome by the invention as it is defined in the claims.